CCD scanner having improved specular reflection discrimination

ABSTRACT

An optical scanner utilizes two linear CCD detectors and a bandpass means to improve the ability of the scanner to discriminate against specular reflection. A coded symbology is illuminated by a noncoherent light source and light reflected from the coded symbology along a first path strikes the front face of the bandpass means. The bandpass means, functioning as a notch filter, transmits a select bandwidth of light while reflecting all other light onto a first CCD detector. Simultaneously, light reflected from the bar code symbol travels along a second path, at a different angle with respect to the plane of the coded symbology than the first path, is reflected from a mirror onto the back face of the bandpass means. The bandpass means transmits the select bandwidth of light onto a second CCD detector and reflects all other light. The second CCD detector has a notch filter which permits the detection of only the select bandwidth. Since specular reflection is only experienced at a single angle, with respect to the plane of the coded symbology and each CCD detector detects an image at a different angle with respect to the plane of the coded symbology, a complete image can be reconstructed by combining information obtained from both CCD detectors.

This application is a continuation of U.S. patent application Ser. No.08/790,956 Jan. 29, 1997, now U.S. Pat. No. 5,942,762.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical scanning systems.More particularly, this invention relates to a system and method capableof detecting coded symbologies in the presence of specular reflection.

2. Description of Related Art

Coded symbologies are being used in an increasingly diverse array ofapplications. The ability to track a large amount of items quickly andefficiently has led coded symbologies to be used in applications such asretail checkout, warehousing, inventory control and document tracking.As the volume of items tracked by coded symbologies has increased, theneed for optical scanners which operate at high speeds has likewiseincreased.

Various optical scanning systems have been developed for reading anddecoding coded symbologies. Scanning systems include optical laserscanners and optical charge-coupled device (CCD) scanners. Optical laserscanners generally employ a laser diode, a multifaceted polygonalmirror, focusing optics and a detector. The scanning rate of an opticallaser scanner is limited by the number of facets on the mirror and theavailable motor speed.

CCD scanners may incorporate a non-laser light source and a CCD lightdetecting means, such as a CCD linear sensor. A portion of the lightwhich is reflected from the coded symbology is detected by the CCDlinear sensor and converted into an electrical signal which is the basisfor a digital image of the coded symbology that has been scanned. Thedigital image is then processed and decoded according to the specifictype of coded symbology.

One disadvantage with current CCD scanners is that they are susceptibleto specular reflection which saturates areas of the CCD linear sensorand prohibits the detection of a portion of the optically codedinformation. This is particularly a problem when the coded symbology isprinted under a highly reflective surface, such as a plastic coating.

Specular reflection is only a problem at a single angle, known as the"critical angle", between the light source, the reflective surface andthe CCD linear sensor. Current methods of coping with specularreflection include placing separate scanners at different angles withrespect to the surface. However, providing duplicate CCD scanners forthis purpose is extremely expensive. Techniques involving lightpolarizers have also been used. However, due to the light lossesintroduced by the materials used to make light polarizers, they areextremely inefficient.

Accordingly, there exists a need for an efficient and inexpensivescanning system with the speed of a CCD scanner that can accurately readand decode coded symbologies in the presence of specular reflection.

SUMMARY OF THE INVENTION

The present invention utilizes two CCD linear sensors and a bandpassmeans to improve the ability of an optical scanner to discriminateagainst specular reflection. A coded symbology is illuminated by anoncoherent light source and light reflected from the coded symbologytravels along a first path and strikes the front face of the bandpassmeans. The bandpass means, functioning as a notch filter, transmits aselect bandwidth of light while reflecting all other light onto a firstCCD linear sensor. Simultaneously, light reflected from the bar codesymbol travels along a second path, at a different angle with respect tothe plane of the coded symbology than the first path, and is reflectedfrom a mirror onto the back face of the bandpass means. The bandpassmeans transmits the select bandwidth of light onto a second CCD linearsensor and reflects all other light. The CCD linear sensors each have anotch filter which permits the detection of only a select bandwidth.Since specular reflection is only experienced at a single angle withrespect to the plane of the coded symbology, and each CCD linear sensordetects an image at a different angle with respect to the plane of thecoded symbology; a complete image of the coded symbology is obtained byone or both of the CCD linear sensors, or can be reconstructed bycombining information obtained from both CCD linear sensors.

Accordingly, it is an object of the invention to provide a CCD scannerwhich can read and decode coded symbologies in the presence of specularreflection.

Other objects and advantages will become apparent to those skilled inthe art after reading the detailed description of a presently preferredembodiment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a coded symbology scanning system made in accordance with thepresent invention;

FIG. 2A is a diagram showing the spectrum of light;

FIG. 2B is a more detailed diagram of the CCD detectors;

FIG. 3 illustrates the method of using valid information from two viewsand selectively combining the information;

FIG. 4 is a block diagram of the coded symbology logic unit;

FIG. 5 is a flow diagram of the method of the present invention; and

FIG. 6 is an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment will be described with reference to the drawingfigures wherein like numerals represent like elements throughout.Referring to FIG. 1, a coded symbology scanning system 10 made inaccordance with the present invention is shown. The coded symbologyscanning system 10 is able to scan any type of coded symbology. However,for simplicity, reference hereinafter will be made to a particular typeof coded symbology, i.e. a bar code symbol. The scanning system 10includes a non-coherent light source 12, a bandpass means 14, a planarmirror 22, focusing optics 17, two CCD linear sensors 16A and 16B, twofilters 19A and 19B, a logic unit 32 and an output means 34.

The light source 12 facilitates detection of a subject bar code symbol18 by illuminating the bar code symbol 18 located on a package 8 orother object. Preferably, the package 8 is supported by a movingconveyor belt 7. The planar mirror 22 and the bandpass means 14 arealigned such that light reflected from the bar code symbol 18 along afirst path 20A strikes the front of the bandpass means 14, while lighttraveling along a second path 20B reflects off the planar mirror 22 andstrikes the rear of the bandpass means art 14. It should be recognizedby those skilled in the art that FIG. 1 is illustrative only and is notdrawn to scale. For example, the angle θ_(A) between the light source 12and the bar code symbol 18 is typically 77°. The angle θ_(B) between thefirst path 20A and the second path 20B is approximately 3-5°. However,it should be recognized by those skilled in the art that these anglesare approximate and may vary widely depending upon the specificapplication and the mounting of the system 10 in relation to the barcode 18.

The bandpass means 14 permits light of predetermined wavelengths aroundη_(A), striking either its front or rear surface, to pass through thebandpass means 14, and reflects the remainder of the light spectrum. Thespectrum of light η_(20A) traveling along the first path 20A strikes thefront of the bandpass means 14. Light having wavelengths around η_(A)passes through the bandpass means 14, while the remainder of thespectrum of light η_(20A) -η_(A) ± is reflected toward the CCD detectors16A, 16B. The spectrum of light η_(20B) traveling along the second path20B is reflected off the planar mirror 22 and strikes the back of thebandpass means 14. Light having wavelengths around η_(A) passes throughthe bandpass means 14 toward the CCD detectors 16A, 16B, while theremainder of the light spectrum η_(20B) -η_(A) ± is reflected off theback of the bandpass means 14.

It should be appreciated that the bandpass means 14 may function as afilter wherein the bandpass means 14 transmits a small bandwidth oflight while reflecting the remainder of the light spectrum.Alternatively, the bandpass means 14 may function as a mirror, whereinthe bandpass means 14 reflects a small bandwidth of light whiletransmitting the remainder of the light spectrum. Preferably a mirroreddichroic filter is used.

The composite spectrum η_(S) of light which reaches the focusing optics17 comprises predetermined wavelengths around η_(A) from the second path20B and the remainder of the spectrum η_(20A) -η_(A) ± from the firstpath 20A. The composite spectrum η_(S) passes through the focusingoptics 17, through the filters 19A, 19B and onto the CCD linear arraydetectors 16A, 16B. Both filters 19A, 19B permit the respective detector16A, 16B to detect non-overlapping bands of light.

Referring to FIG. 2, the second CCD detector 16B is filtered to detectlight having wavelengths around η_(A). The first CCD detector 16A isfiltered to permit the detection of light around a different wavelengthη_(B). For example, the bandpass means 14 may be calibrated to transmitlight around wavelength η_(A) of 650 NM±. The second CCD detector 16B isfiltered to detect light around the wavelength η_(A) of 650 NM±originating from the second path 20B. The first CCD detector 16B isfiltered to detect light around wavelength η_(B) which originates fromfirst path 20A, for example 600 NM±. Accordingly, the detectors 16A, 16Bwill detect two separate images of the bar code symbol 18.

Although the detectors 16A, 16B have been referred to as separate CCDlinear sensors, preferably they comprise two of the three channelscommonly found in a color CCD line scan sensor. In this embodiment, thecolor filters are preferably replaced with the appropriate notch filters19A, 19B. Those of skill in the art should realize that the bandwidthtransmitted by each notch filter 19A, 19B, including tolerances, shouldnot overlap with the other notch filter 19A, 19B. Additionally, thenotch filters 19A, 19B need not be of equal bandwidth. One notch filter19A may have a narrow bandwidth of 590-610 NM±, and the other notchfilter 19B may have a wide bandwidth of 625-675 NM±. Additionally,although two notch filters 19A, 19B may be employed, it is also possibleto use one notch filter 19A, wherein the other filter 19B transmits allother wavelengths of light except for the bandwidth transmitted by thenotch filter 19A. In all of these examples, the tolerances of thefilters 19A, 19B should be kept in mind to avoid any overlap.

It should be apparent to those skilled in the art that the bandpassmeans 14 and the filters 19A, 19B over the CCD detectors 16A, 16B may becalibrated to detect any wavelength of light that is suitable for thedesired application. The above values are illustrative only and shouldnot be viewed as a limitation of the invention.

The light detected by the second CCD detector 16B comprises light fromthe second path 20B having wavelengths around η_(A). The light detectedby the first CCD detector 16A comprises light from the first path 20Ahaving wavelengths around η_(B). By definition, specular reflectionsonly occur at a "critical angle". Once specular reflection occurs, thisangle is defined and will be present only in one of the optical paths.Therefore, the other path will have useful information. If specularreflection "washes out" the view of the bar code symbol 18 at any pointalong the first path 20A, specular reflection will not be present in thesecond path 20B at the same point since the angle of the bar code symbol18 with respect to the second path 20B is different than the angle withrespect to the first path 20A.

Referring to FIG. 2B, since the lengths of the two paths 20A, 20B aredifferent, the detectors 16A, 16B must be selectively placed to accountfor this difference. In FIG. 1, path 20A is shorter than path 20B.Preferably, the detectors 16A, 16B are mounted upon a common substratewhich is rotated upon a center line CL to position the first detector16A further from the focusing optics 17 than the second detector 16B.

Each of the CCD detectors 16A, 16B produces an electrical signal whichcorresponds to the detected light. Using the images 30A, 30B, 30C inFIG. 3 as a visual example of the reconstruction process, comparison ofimages 30A and 30B shows that image 30A has portions of specularreflection distortion, while image 30B also has portions of specularreflection distortion. However, the non-distorted areas of the images30A, 30B can be used to form the complete image 30C. Although the images30A, 30B, 30C are illustrated as area images, the preferred embodimentof the present invention detects and combines multiple line scans whichmake up the area images. It is clearly within the scope of the presentinvention to utilize detectors which detect either line or area scans.

Processing of the data from CCD detectors 16A, 16B to construct acomplete bar code symbol 18 will be explained with reference to FIG. 4.The data from the CCD detectors 16A, 16B is output and analyzed by thelogic unit 32. Depending upon the amount of specular reflection, datafrom one or both of the CCD detectors 16A, 16B may comprise a completeimage of the bar code symbol 18. In that case, the complete image isused for further decoding in accordance with the specific type ofsymbology. If specular reflection is detected by the logic unit 32 inthe data output from the first CCD detector 16A the logic unit 32replaces the data with the data from the second CCD detector 16B.

Referring to FIG. 4, the logic unit 32 comprises two buffers 70A, 70B, aselector 72 and an arbitration unit 74. The logic unit 32 receives thedata, containing bar code information, from the CCD detectors 16A, 16B.The information coming from the CCD detectors 16A, 16B is selectivelybuffered depending upon the height of the package 8 upon which the barcode 18 is affixed. Referring back to FIG. 1, at a first height Y, theinformation is obtained simultaneously from both light paths 20A, 20B.Accordingly, no buffering of the data is required. However, when thepackage 8 to which the bar code 18 is affixed reaches height X, thesecond light path 20B will obtain the bar code information prior to thefirst light path 20A. Therefore, information from the second light path20B must be buffered by the buffer 70B prior to comparison with theinformation from the first light path 20A. Conversely, if the height ofthe package 8 to which the bar code 18 is affixed only reaches height Z,the first light path 20A will detect the information prior to the secondlight path 20B. In this event, the information from the first light path20A will be buffered by buffer 70A. Each buffer 70A, 70B delays theinformation obtained from the respective light path 20A, 20B tosynchronize the information with that obtained from the other light path20B, 20A.

As discussed above, the delay is dependent upon the distance between thesystem 10 and the bar code symbol 18. The distance between the system 10and a package 8 having the bar code symbol 18 located thereon may beobtained by using a light curtain 9, as in FIG. 1, or by any other meanswhich is well known by those skilled in the art. From the height, ordistance, the delay value may be calculated, or a look up table may beused. The delay value is then input into the desired buffer 70A, 70B.

After the signal output from either detector 16A, 16B has been bufferedas necessary, the signals are compared by the arbitration unit 74. Thesignals comprise values which represent the intensity of light detectedby the pixels of the CCD detectors 16A, 16B. If the CCD detectors 16A,16B have eight-bit resolution, the number of gray scale levels will be255 (2⁸ -1). Depending upon the application, it may be assumed that avalid signal will have a gray scale value between 0 and 240. If the grayscale value exceeds a predetermined threshold of 240, specularreflection is present. This threshold may be adjusted depending upon theparticular application. In the preferred embodiment the arbitration unit74 controls the selector 72 to select the output from the second CCDdetector 16B when the value from the output from the first CCD detector16A exceeds 240. In this manner, a complete image of the bar code symbol18 is obtained.

The logic unit 32 forwards a complete digital image, corresponding tothe information encoded in the bar code symbol 18, to an image processor34 for decoding, storage or display, as is well known by those skilledin the art.

The scanning system 10 shown in FIG. 1 may be embodied in a mobilehand-held unit, or may be a stationary unit wherein an object carryingthe bar code symbol 18 is passed under the light source 12 via aconveyor 7.

In operation, the scanning system 10 executes the bar code symbolreading and decoding procedure 200 shown in FIG. 4. The light source 12illuminates a subject bar code symbol 18 (step 210). Light is reflectedfrom the bar code symbol 18 along a first path 20A toward the front ofthe bandpass means 14 (step 220). The bandpass means 14 transmits lightaround a first predetermined wavelength (step 230) and reflects theremainder of the light spectrum toward the CCD detectors 16A, 16B (step240). The first CCD detector 16A detects light around a secondpredetermined wavelength from the first light path 20A. (step 250).

Simultaneously, light is reflected from the bar code symbol 18 along asecond path 20B (step 270) toward the back of the bandpass means 14(step 280). The bandpass means 14 passes light around the firstpredetermined wavelength to the CCD detectors 16A, 16B (step 290) andreflects the remainder of the light spectrum away from the CCD detectors16A, 16B (step 300). Light originating from the second path 20Bcomprises only light around the first predetermined wavelength.Accordingly, it will be detected by the second CCD detector 16B (step310).

The CCD detectors 16A, 16B convert the detected light into electricalsignals which are output to the logic unit 32 (steps 260, 320). Thelogic unit 32 compares the signals (step 330) and the valid data isselected (step 340). This data is used to provide a complete bar codesymbol 18. In the event that both signals comprise non-distorted data,the non-distorted data of either signal may be used. The logic unit 32then arbitrates the data representing the complete bar code symbol 18(step 350) and forwards the data to the output means 34 (step 350).

Referring to FIG. 6, an alternative embodiment of the scanning system110 is shown in which additional mirrors 124 and 126 are added to thesystem 110 to direct the paths of light along a modified route. Themodified route permits alignment of the components in cases wheremanufacturing or other considerations require that the components beplaced in a configuration other than that shown in FIG. 1. It should beunderstood that various additional components and configurations can beemployed to alter the light paths and the intensity and precision of thelight without departing from the spirit and scope of the invention.

Although the invention has been described in part by making detailedreference to the preferred embodiment, such detail is intended to beinstructive rather than restrictive. It will be appreciated by thoseskilled in the art that many variations may be made in the structure andmode of operation without departing from the teachings herein.

What is claimed is:
 1. A scanner of a type that utilizes reflectedillumination from the surface of an object for scanning a symbology, thescanner comprising:a detector that collects illumination reflected fromthe symbology at different angles of reflection and outputs a detectorsignal wherein the light collected from each angle of reflection hasdifferent spectral content from light collected at other angles; and alogic unit that receives the detector output signal, evaluates therelative intensities of the reflected illumination and discriminatesbetween the reflected illumination based on the evaluation of relativeintensities.
 2. The scanner of claim wherein the detector includesmultiple light sensors.
 3. The scanner of claim 1, wherein the lightdetected at the first angle travels along a first path defined by abandpass means; and light detected at the second angle travels along asecond path defined by a mirror, with the first and second pathsconverging prior to being incident upon the detector.
 4. The scanner ofclaim 2 wherein the detector is angled with respect to light incidentthereupon.
 5. The scanner of claim 4 wherein the detector is orthogonalto light incident thereupon.
 6. The scanner of claim 3 wherein thebandpass means is a filter.
 7. The scanner of claim 6, wherein thedetector further comprises a second bandpass means positioned to receivelight from the bandpass means.
 8. The scanner of claim 7 wherein a firstlight sensor is positioned adjacent a front surface of the secondbandpass means and a second light sensor is positioned adjacent a backsurface of the second bandpass means.
 9. The scanner of claim 6, whereinthe bandpass means is a mirrored dichroic filter.
 10. The scanner ofclaim 6, wherein the bandpass means is a notch filter.
 11. The scannerof claim 10, wherein the detector includes multiple light sensors. 12.The scanner of claim 11 wherein the bandpass means passes only apredetermined band of wavelengths from the light of the first angle to afirst sensor and reflects all other wavelengths.
 13. The scanner ofclaim 12 wherein the bandpass means passes the predetermined band ofwavelengths from the light of the second angle and reflects all otherwavelengths toward a second sensor.
 14. The scanner of claim 13 whereinsaid first light sensor includes a first notch filter and said secondlight sensor includes a second notch filter; wherein said first notchfilter transmits a first predetermined bandwidth of light and saidsecond notch filter transmits a second predetermined bandwidth of light.15. The scanner of claim 14 wherein the first notch filter transmitslight from the first path and the second notch filter transmits lightfrom the second path.
 16. The scanner of claim 15 wherein both the firstand second sensors generate an electrical signal corresponding to theintensity of light incident thereupon.